1. Field of the Invention
The present invention relates to gypsum calcination, and more particularly to a process for continuous kettle calcination of chemical gypsum to beta hemihydrate useful in the formulation of building plasters and in gypsum wallboard manufacture.
Chemical gypsum, calcium sulfate dihydrate resulting from an acid neutralization step in any of numerous manufacturing operations, differs from natural mineral gypsum in its chemical impurities and, frequently most importantly, in physical characteristics. Chemical impurities from the production process affect calcination properties and the setting times and strength development of the resultant stucco. Chemical gypsums are generally obtained as very uniform, complete crystallization crystals, particles of very narrow particle size distribution (generally 800-2000 square centimeters per gram Blain surface areas, and mean particle size about 50 micrometers and the majority of the particles are between 10 and 70 micrometers). This affects calcination properties including lengthy times for calcination heat to penetrate the relatively large crystals and only gradual effecting of complete calcination due to the narrow particle size distribution.
Further, the free water associated with the as-produced chemical gypsum cake greatly affects flow and handling properties of the chemical gypsum. Completely dry chemical gypsum flows readily. Chemical gypsum with greater than about 40% free water becomes very free flowing, virtually acting like a fluid. However, wet chemical gypsum with associated free water in between these contents is a virtually immovable, sticky mass. Due to this difficulty with flow and handling chemical gypsum which is to be calcined, as a practical matter, is subjected to a preliminary step of either being slurried with large quantities of water for use in greater than atmospheric pressure calcination to produce alpha hemihydrate, or it is thoroughly dried to less than 1% free water content before attempting atmospheric calcination to produce beta hemihydrate.
2. Description of the Prior Art
This invention treats the gypsum feed with water before calcination as opposed to prior art practices which treat after calcination.
U.S. Pat. No. 1,548,358 pertaining to a batch atmospheric kettle calcination of phosphogypsum, does contain a suggestion that the chemical gypsum might not have to be completely dried as a prerequisite step. That suggestion, however, is for conditions under a batch atmospheric calcination wherein the cold calcination vessel is filled with cold (ambient temperature) materials, gradually heated for a gentle calcination in the uncovered vessel, and when calcination is complete the contents of the kettle are dumped completely before the start of another cycle.
In the more modern continuous fluidized kettle calcination, as exemplified by U.S. Pat. No. 3,236,509, dried mineral gypsum powder is fed to a covered, but air vented and lightly vacuum exhausted, calcination vessel. After a steady state of operation is attained in the vessel, a substantially continuous stream of cold gypsum that has been pre-dried and ground to a finely divided state and with a wide distribution of fragmented particle sizes, is added on top of the fluidized, boiling mass in the kettle. Under such conditions, the thermal shock upon the cold, dry mineral being dropped into the already boiling mass radically fractures the ground gypsum rock fragments, and the resultant stucco (beta hemihydrate) is highly fractured and fissured, as well as being widely distributed in particle size. This causes the stucco to disperse very rapidly in water, and requires high amounts of gauging water to be mixed with the stucco for rehydration to gypsum at customary dispersed consistencies. For purposes of the present invention "dispersed consistency" may be defined as the water volume required to give a standard viscosity or flow when a standard amount by weight of stucco is dispersed by mechanical mixing in a laboratory mixer at high shear intensity and for a standard time to equal mixing encountered in the gypsum board forming line, e.g., 7 seconds, or in an industrial plaster formulation casting mixer, e.g. 60 seconds.
Post calcination methods to improve the properties of stucco from continuous kettle calcination are also known. Thus, U.S. Pat. No. 4,201,595 and related patents teach means to lower the dispersed consistencies of continuously calcined kettle stuccos by an after calcination treatment of the stucco with small amounts of water or various aqueous solutions. These methods require an attendant capitalization cost to provide a healing time for the treated stucco.
Further, it has been common practice for many years to mix proportions of thoroughly dried chemical gypsum containing less than 1% free water into the dry mineral gypsum fed in an atmospheric pressure continuous kettle calcination, e.g. up to about 35% by weight dry chemical gypsum is added to the finely ground rock fragments of widely varying particle size distribution. The calcination of such blends results in a hemihydrate requiring slightly less amounts of gauging water for dispersed consistencies than for mineral gypsum feed.
It is believed that no one has heretofore thought to feed the wet sticky chemical gypsum materials to continuous atmospheric pressure calciners, since such would merely increase energy requirements in evaporating the free water before beginning to calcine the gypsum. In addition, the extreme difficulties in handling wet chemical gypsum would lead persons skilled in the art away from the calcination of wet chemical gypsum.